IBM 1130 User Manual page 100

Section
Subsections
Page
15 20
I
60 01
Always Back Up Your Disk Files with a Duplicate
Copy
Regardless of how the processing system is de-
signed, there should be a duplicate copy of every
disk data file. If you have multiple disk drives,
you can copy from one disk drive onto another; if
you have one drive, you must dump to cards. The
copying (or dumping) should be on a regular basis,
and should not be left to chance or done whenever
there is nothing else to do. Both copying and dump-
ing may be done easily with the Disk Utility Pro-
gram, as outlined in section 60.
If your 1130 system has only one disk drive, it is
impossible to copy disks, and backup must be in the
form of cards. Either the DUP *DUMPDATA func-
tion may be used, or you may write your own dump
program. With large data files, both dumps take a
significant amount of time. For example, it takes
about three hours to dump a 1000-sector data file.
Because of the time involved, there is a natural
tendency to avoid dumping such files. However, an
analysis of a typical situation shows this to be self-
defeating.
Assume an 800-man employee file, contained in
400 sectors. To dump it with a 1442, Model 6,
takes about 60 minutes.
The weekly processing sequence is as suggested
earlier:
PAY16 Edit 30 min.
PAY04 Calculations, Disk Update 90 min.
and Payroll Register
PAY05 Payroll Checks 60 min.
PAY06 Check Register 30 min.
For purposes of analysis, assume the worst pos-
sible case - namely, that somehow during PA Y04
the payroll data cartridge is completely destroyed.
(No matter how improbable or infrequent you think
this might be, it can happen.) How do you recover?
If you dump the data file every week, you must:
Reload the dumped deck 30 minutes
Rerun PA Y16 and PAY04 2 hours
You have completely recovered in 2-1/2 hours.
If you dump every other week, and you again con-
sider the worst case (your last dump was two weeks
ago), you must get the data cards from last week,
and:
Reload the dumped deck
Rerun PA Y16 and PA Y04 with
last week's cards
Rerun PAY16 and PA Y04 with
this week's cards
30 minutes
2 hours
2 hours
In
4-1/2 hours you have caught up to where you were
before the accident.
If it had been three weeks since your last dump,
the reconstruction time would be 6-1/2 hours; four
weeks, 8-1/2 hours; five weeks, 10-1/2 hours; etc.
Each week adds about 2 hours.
These figures assume that you can immediately
lay your hands on the previous week's data cards in
the proper order. If this is not so, these times
could go up drastically. The figures also assume
that everything goes smoothly during the recovery
phases. This, however, is not a very safe assump-
tion, since the operators will be rushed and unfamil-
iar with the procedures.
Without knowing the probability of such an acci-
dent, it is impossible to compute the optimum dump
frequency. It is probable, however, that you will
not want to be in a 10-1/2-hour recovery position,
no matter how slight the probability, just to save an
hour a week and a few thousand cards.
In this case, the best approach would seem to be
a dump every week for the first few months of the
installation, every other week after everything has
stabilized, and every third week if conditions seem
to warrant it.